1. Technical Field of the Invention
This disclosure relates in general to oscillators, and more particularly, to a nonvolatile programmable crystal oscillator.
2. Description of the Related Art
A crystal oscillator, or XO, is an electronic device that uses the mechanical resonance of a physical crystal of piezoelectric material to create an electrical signal with a very precise frequency. The XO is an especially accurate form of an electronic oscillator, and it is the most common source of time and frequency signals. The piezoelectric crystal of an XO is typically composed of synthetic (man-made) quartz, but it may also be made of rubidium or ceramic. The output frequency of the XO may be used to, among other things, keep track of time (as in quartz wristwatches) or to stabilize frequencies for radio transmitters. Since the piezoelectric crystals may also be embedded in integrated circuits, the XOs are also frequently used to provide a stable clock for digital circuits.
The piezoelectric crystal in the XO may also be referred to as a “timing crystal,” or “resonator.” Because of the piezoelectric effect, the resonator strains (expands or contracts) when an electrical voltage is applied. When the applied voltage is reversed, the strain is reversed. The above application of voltages causes the resonator to oscillate.
Oscillation of the XO is sustained by taking a voltage signal from the resonator, amplifying it, and feeding it back to the resonator. The rate of expansion and contraction of the timing crystal is the resonance frequency, and it is a function of the cut and the size of the crystal. The output frequency of an XO is either the resonance frequency or a multiple of the resonance frequency, called an overtone frequency.
The XO is an important device because of its ability to have extremely narrow bandwidth with good filter shape factor (sharpness of passband/stopband characteristic). At their frequencies of operation, XOs can generate narrow bandwidths, unobtainable with lumped element inductors (Ls) and capacitors (Cs). The quality factor (Q) is the parameter that describes this performance. By definition, Q is the ratio of energy stored by oscillation cycle to energy lost per cycle. A typical Q for an XO such as a quartz oscillator ranges from 104 to 106. The maximum Q for a high stability quartz oscillator can be estimated as Q=1.6×107/f, where f is the resonance frequency in MHz.
The short-term stability of the output of the XO is affected by environmental changes such as temperature, humidity, pressure, and vibration, all of which can change the resonance frequency of the timing crystal. The long-term stability of the XO is primarily affected by aging of the crystal itself.
Due to aging and environmental factors such as temperature and vibration, it is hard to keep even the best XOs within 10−10 of their nominal frequency without constant adjustment.
As is well-known in the art, XOs may be classified according to the methods by which their frequency outputs are controlled. For instance, XOs may be classified as voltage-controlled crystal oscillators (VCXOs), temperature-compensated crystal oscillators (TCXOs), oven-controlled crystal oscillators (OCXOs), temperature-compensated-voltage controlled crystal oscillators (TCVCXOs), oven-controlled voltage-controlled crystal oscillators (OCVCXOs), or microcomputer-compensated crystal oscillators (MCXOs). In U.S. Pat. No. 6,563,390, a digitally compensated Voltage-Controlled Oscillator (VCXO) is disclosed. The digitally compensated VCXO may be adjusted with a tuning circuit that includes a frequency tuning array that is implemented with a non-volatile memory.
FIG. 1 is a block diagram illustrating some components of a conventional crystal oscillator circuit. The oscillator circuit 100 includes an XO 101, a Phase-Locked Loop (PLL) 102, and a Non-Volatile Memory (NVM) 103. The possible internal arrangements of the oscillator 101, PLL 102, and NVM 103 are numerous and well-known to those of skill in the art.
In crystal oscillator circuit 100, the NVM 103 receives external configuration signals (CONFIG) as input. Based upon the configuration signals, the NVM 103 controls the XO 101 and the PLL 102. The XO 101 produces a reference frequency FREF, which is input to the PLL 102. In a well-known process, the PLL 102 multiplies the reference frequency FREF by a ratio of integers to achieve the desired output frequency FOUT. The ratio of integers may be greater than or less than 1.
When a conventional crystal oscillator circuit, such as circuit 100 as described above, is implemented in an integrated circuit, the divider registers of the PLL 102 must be programmed at metal mask. That is, existing Read-Only Memory (ROM) technologies such as trim, select-at-test, or metal mask options are performed to configure the PLL 102. Metal options increase the mask cost for the product, while trim technologies may only be applied once per die at wafer sort, rendering the dies suitable for only one particular application.
Embodiments of the invention improve upon these and other features of the above-described art.